Contextual modulation in primary visual cortex of macaques.

Recent studies have suggested that V1 neurons extract figures from their backgrounds, in that they respond better to interior features of figures than to equivalent features of background stimuli. This is reportedly true even when the figure boundaries are distant from the borders of the classical receptive field (RF). To test the role of V1 neurons in figure-ground segregation, we recorded their responses to texture figures on texture backgrounds, centered on the RF. The texture elements of the figures remained identical across trials, and figure boundaries were defined by orientation differences between the elements in the background texture relative to elements in the figure. For nearly all neurons (98/102), responses to a large texture figure did not differ from the responses to a uniform-texture background. Although many neurons gave enhanced responses to texture boundaries, this occurred only when the boundaries were within or close to the RF borders. Similar effects were found in V2. For neurons in V1, the limited spatial extent of the contextual modulation was not increased either at low stimulus contrast or when the animal was rewarded for detecting an orientation-defined figure. Thus, V1 neurons appear to signal texture boundaries rather than figures per se. Unexpectedly, many V1 neurons gave significant long-latency responses to texture stimuli located entirely outside the classical RF, up to 5 degrees from the RF border in some cases. However, these responses did not depend on the stimulus forming a figure that contained the RF. Although V1 neurons are influenced by stimuli outside the classical RF, they do not appear to segregate figures from ground.

Cardiac arrest in infants after congenital heart surgery.

BACKGROUND: The survival rate to discharge after a cardiac arrest in a patient in the pediatric intensive care unit is reported to be as low as 7%. The survival rates and markers for survival strictly regarding infants with cardiac arrest after congenital heart surgery are unknown. METHODS AND RESULTS: Infants in our pediatric cardiac intensive care unit database were identified who had a postoperative cardiac arrest between January 1994 and June 1998. Parameters from the perioperative, prearrest, and resuscitation periods were analyzed for these patients. Comparisons were made between survivors and nonsurvivors. Of 575 infants who underwent congenital heart surgery, 34 (6%) sustained a documented cardiac arrest; of these, 14 (41%) survived to discharge. Perioperative parameters, ventricular physiology, and primary rhythm at the time of arrest did not influence outcome. Prearrest blood pressure was lower in nonsurvivors than in survivors (P<0.001). A high level of inotropic support prearrest was associated with death (P=0.06). Survivors had a shorter duration of resuscitation (P<0.001) and higher minimal arterial pH (P<0.02) and received a smaller total dose of medication during the resuscitation. Although survivors had an overall shorter duration of resuscitation, 5 of 22 patients (23%) survived to discharge despite resuscitation of >30 minutes. CONCLUSIONS: The outcome of cardiac arrest in infants after congenital heart surgery was better than that for pediatric intensive care unit populations as a whole. Univentricular physiology did not increase the risk of death after cardiac arrest. Infants with more hemodynamic compromise before the arrest as demonstrated with lower mean arterial blood pressure and higher inotropic support were less likely to survive. The use of predetermined resuscitation end points in this subpopulation may not be justified.

Prolongation of the QT interval in children with liver failure.

BACKGROUND: Alcoholic liver disease has been associated with QT prolongation and sudden cardiac death. HYPOTHESIS: We evaluated children with hepatic failure to determine whether they have abnormalities of ventricular repolarization. METHODS: Between October 1990 and January 1996, 38 pediatric patients (mean age 6.5 +/- 7.2 years) underwent evaluation for liver transplantation, including a 12-lead electrocardiogram and an echocardiogram. All patients had normal serum electrolytes, calcium, and magnesium at the time of cardiac evaluation and were not on any medications known to prolong repolarization. Follow-up electrocardiograms were performed on all survivors with QT prolongation following liver transplantation. RESULTS: Among those evaluated, seven (18%) were noted to have a prolonged QT interval corrected for rate (QTc > 450 ms; range 460-560 ms). All had a structurally normal heart, except one with an atrial and ventricular septal defect. When compared with patients with a normal QT interval, there was no significant difference in serum indices of liver function or indication for liver transplantation. None of the patients developed a ventricular arrhythmia. Two patients with a prolonged QTc died prior to transplant and another died immediately after surgery. All four survivors had normalization of the QTc following liver transplantation. CONCLUSION: QTc prolongation can be seen in a significant number of children with hepatic failure. While the mechanism is not known, it appears to be reversible following liver transplantation.

Neural correlates of perceived brightness in the retina, lateral geniculate nucleus, and striate cortex.

Brightness changes can be induced in a static gray field by modulating the luminance of surrounding areas. We used this induction phenomenon to investigate the neural representation of perceived brightness. Extracellular recordings were made in striate cortex, the lateral geniculate nucleus (LGN), and the optic tract of anesthetized cats using stimuli that produced brightness induction. While a cell's receptive field (RF) was covered by uniform gray illumination, the luminance of rectangular flanking regions was modulated sinusoidally in time, inducing brightness changes in the RF. We looked for a correspondence between the modulation of a cell's response and stimulus conditions that did or did not produce perceptual changes in brightness. We found that the responses of retinal ganglion cell axons in the optic tract were never correlated with brightness. On the other hand, many neurons in striate cortex and a small fraction in the LGN responded in a phase-locked manner at the temporal frequency of the flank modulation, even though the flanks were 3-7 degrees beyond the edges of the RF. Only in striate cortex were cells found that had responses correlated with brightness under all stimulus conditions. These findings suggest that brightness information is explicitly represented in the responses of neurons in striate cortex as part of a neural representation of object surfaces.

Oxygen transport in critically ill infants after congenital heart operations.

BACKGROUND: Oxygen transport variables reflect the balance of oxygen delivery and demand. Because oxygen transport in infants undergoing congenital cardiac operations is not well described, we examined oxygen transport in such patients. Differences in oxygen transport between survivors and nonsurvivors and variables that might be predictive of outcome were sought. METHODS: We reviewed hospital records of infants admitted to the pediatric cardiac intensive care unit in our institution from January 1996 through April 1997. Infants in whom simultaneous arterial blood gas and systemic venous oxygen saturation measurements were performed on admission and at 6 and 24 hours after admission were included. Analyses of arterial pH, base excess, arteriovenous oxygen saturation differences, and oxygen extraction ratio were performed, including comparisons of survivors and nonsurvivors and changes over time. RESULTS: Forty-nine infants were included in the study, with 39 survivors. There were no differences in any parameter between survivors and nonsurvivors on admission or at 24 hours. At 6 hours, differences between survivors and nonsurvivors were significant for arterial pH (7.48 versus 7.35, p<0.001), base excess (2.9 versus -4.3 mmol/L, p<0.01), arteriovenous oxygen saturation difference (34 versus 43, p<0.05), and oxygen extraction ratio (0.28 versus 0.53, p<0.001). The oxygen extraction ratio at 6 hours was at least 0.5 in 6 of 39 survivors and 7 of 10 nonsurvivors (p = 0.002). CONCLUSIONS: Infants who die after cardiac operations have significant derangements of oxygen transport at 6 hours after admission to the intensive care unit. Infants with an oxygen extraction ratio greater than 0.5 at 6 hours are at highest risk.

Successful radiofrequency catheter ablation of congenital junctional ectopic tachycardia with preservation of atrioventricular conduction in a 9-month-old infant.

An infant with congenital junctional ectopic tachyardia required frequent hospitalizations due to tachycardia acceleration despite multiple antiarrhythmic medications. At 9 months of age, he underwent successful radiofrequency catheter ablation of the tachycardia with preservation of AV conduction.

Usefulness of tacrolimus versus cyclosporine after pediatric heart transplantation.

This study compared the early clinical course of 9 pediatric heart transplantation recipients treated with cyclosporine A-based immunosuppression with 10 similarly aged recipients treated with tacrolimus-based therapy. One-year follow-up after transplantation revealed that tacrolimus-treated children had similar left ventricular function, experienced fewer episodes of severe rejection, were more rapidly weaned from corticosteroids, and had relatively few side effects from immunosuppression compared with cyclosporine A-treated children.

Long-term efficacy of endoscopic stenting in patients with stricture of the biliary anastomosis after orthotopic liver transplantation.

BACKGROUND AND STUDY AIMS: The choledocho-choledochostomy (CCS) stricture is one of the most frequent complications occurring after liver transplantation. Endoscopic retrograde cholangiography (ERCP) is the most sensitive method used to define the presence and narrowness of the stricture. Endoscopic stenting of the strictured anastomosis could provide an effective alternative to the surgical intervention. PATIENTS AND METHOD: ERCP was performed in 36 of 210 patients with liver transplantation and acute cholestasis or jaundice: in 15 cases biliary anastomotic stricture was found. These patients were endoscopically treated by long-term stenting of the common bile duct (CBD) (1 year) and followed up for more than 12 months after stent removal. RESULTS: In all cases the stenting procedure resolved the biliary obstruction syndrome within 7 days. At the end of the stenting period the CCS was dilated enough to allow adequate bile flow and absence of cholestasis. Moreover, in most patients (10) the anastomosis was kept patient for more than 1 year after stent removal, whereas only two patients had stricture recurrence and needed endoscopic restenting. Four patients dropped out of the study, respectively because of liver rejection (two), acute liver failure (one) and myocardial infarction (one). One patient who developed a stone of the transplanted CBD underwent surgical intervention. CONCLUSIONS: According to our data, the endoscopic stenting of the CBD might be considered as the first choice procedure in the setting of the biliary anastomotic strictures occurring after liver transplantation. It has proved to be safe and effective, avoiding the need for more invasive surgery, which in any case should be considered for nonresponsive patients.

The outcome of cardiac operations in infants weighing two kilograms or less.

OBJECTIVE: A review of our recent experience of operating on infants weighing 2 kg or less who had congenital heart disease was performed to determine the outcome of early surgical repair or palliation. METHODS: A retrospective review of hospital records was performed for infants who weighed 2 kg or less and who were identified to have undergone cardiac operation at our institution January 1992 to June 1997. The data collected included age, weight, gestational age, cardiac diagnosis, surgical procedure, and outcome measures such as length of stay, morbidity, and mortality rate. Outpatient charts were reviewed for follow-up survival and cardiac status. RESULTS: Thirty-three operations were performed on 30 patients. Median age at operation was 19.5 days (1 to 140 days), and median weight was 1.8 kg (1.1 to 2.0 kg). Cardiac diagnoses varied, with coarctation of the aorta and tetralogy of Fallot most common. Twenty-four patients were born at 37 or fewer weeks' gestation. Hospital survival was 83% with no difference in mortality rates based on age, weight, or type of surgical procedure. Premature infants tended to have worse hospital survival. Median postoperative length of stay was 39 days (6 to 122 days). Median duration of mechanical ventilation in survivors was 6 days (2 to 24 days). Neurologic complications were documented in eight patients. Of the 25 hospital survivors, 20 (80%) are alive with good cardiac status at a mean follow-up of 13 months. CONCLUSION: Cardiac operations in a selected group of infants weighing 2 kg or less can provide acceptable hospital survival. In most instances, complete repair is possible with good medium-term outcome in the survivors. Investigation into neurologic outcomes in these patients is warranted.

Variable effects of adenosine on retrograde conduction in patients with atrioventricular nodal reentry tachycardia.

Adenosine has been demonstrated to reliably produce transient block of atrioventricular nodal (AVN) conduction, and has been advocated as a method of differentiating retrograde conduction via the atrioventricular node from accessory pathway conduction. However, the response of retrograde AVN to adenosine in patients with typical atrioventricular nodal reentry tachycardia (AVNRT) remains unclear. We evaluated 13 patients (mean age 45 +/- 20 years) with typical AVNRT prior to AVN modification. During right ventricular pacing, a rapid bolus of adenosine (0.2 mg/kg; maximum 18 mg) was administered. Adenosine sensitivity, defined by transient ventriculoatrial block, was observed in six patients, while in seven patients ventriculoatrial conduction was unaffected. An adenosine bolus administered during sinus rhythm or atrial pacing resulted in antegrade atrioventricular block in all the adenosine resistant patients in whom this was performed (n = 6). Comparisons of AVN electrophysiological characteristics between the adenosine sensitive and adenosine resistant patients were performed. There was no difference with respect to ventriculoatrial effective refractory period, ventriculoatrial Wenckebach, AVNRT cycle length, and His to atrial echo interval in AVNRT. However, there was a trend toward a longer antegrade fast pathway ERP in the adenosine sensitive group (P = 0.07). Electrophysiological properties do not predict retrograde AVN adenosine sensitivity. Adenosine does not cause retrograde AVN block in all patients with AVNRT, and therefore cannot reliably distinguish between retrograde conduction via the AVN or an accessory pathway.

The effects of HLA mismatching and immunosuppressive therapy on early rejection outcome in pediatric heart transplant recipients.

BACKGROUND: Although HLA-DR antigen mismatching between heart transplant recipients and donors has been associated with increased early allograft rejection in adult patients treated with cyclosporine, little information exists in the pediatric age group. In this study we examined retrospectively the effects of HLA mismatching and immunosuppression choice, cyclosporine versus tacrolimus on early rejection outcome in pediatric heart transplant recipients. METHODS: Between 1992 and 1997, 38 patients (ages 10 days to 18 years) underwent 40 heart transplantations. All recipients were typed prospectively and donors retrospectively by use of serologic microcytotoxicity testing for HLA-A and HLA-B antigens and by a polymerase chain reaction technique for HLA-DR antigens. All heart transplant recipients received induction immunosuppression with methylprednisolone and maintenance prednisone, and 38 received OKT3. The first 25 heart transplant recipients received cyclosporine and azathioprine, and the last 15 were given tacrolimus. Clinical courses, HLA mismatching, and biopsy results for the first year after heart transplantation were reviewed and compared between treatment groups. RESULTS: Mean age, donor/recipient weight ratios, and biopsies/patient were similar between treatment groups. Five deaths occurred among cyclosporine-treated patients and none among tacrolimus-treated patients during the study period. HLA mismatching was similar between groups, with 94% of patients having 1 or 2 HLA-A mismatches and 96% having 1 or 2 HLA-B and -DR mismatches. Both International Society for Heart and Lung Transplantation grade 2 and grade 3 or 4 rejections were significantly increased in biopsies from cyclosporine-treated patients (P < .05). Significantly increased grade 3 or 4 rejection was present in patients treated with cyclosporine who had two DR mismatches versus those with one DR mismatch (3.0+/-1.6 vs 1.4+/-0.8; P < .05); no statistical significance between patients treated with tacrolimus with 1 vs 2 DR mismatches was noted. Patients treated with tacrolimus who had 2 DR mismatches had fewer grade 3 or 4 rejection episodes/patient than either patients treated with cyclosporine who had one DR mismatch (0.6+/-0.4 vs 1.4+/-0.8, P = .03) or those treated with cyclosporine who had two DR mismatches (0.6+/-0.4 vs 3.0+/-1.6, P = .01). Grade 3 or 4 rejection episodes/patient were not affected by HLA-A or B mismatching, and grade 2 rejection was not affected by mismatching at any of the loci. CONCLUSION: Although mismatching of HLA-A and -B antigens did not affect frequency of early cellular rejection, the presence of 2 HLA-DR loci mismatches increased the risk of high-grade rejection in pediatric heart transplant recipients treated with cyclosporine. The potent effects of tacrolimus-based immunosuppression mitigated the impact of HLA-DR mismatching, because patients treated with tacrolimus who had 2 DR mismatches had less rejection than even patients treated with cyclosporine who had one DR mismatch and seemed to be at no greater risk for rejection than patients treated with tacrolimus who had 1 DR mismatch.

Use of single lead VDD pacing in children.

The development of transvenous ventricular pacing leads with proximal electrodes capable of atrial sensing and the recent availability of smaller generators has created the opportunity to treat children with complete AV block and normal sinus node function with a transvenous single lead VDD pacing system. Studies in adults have demonstrated this system to be efficacious with low complication rates. Transvenous single lead VDD pacemakers were implanted in ten children, aged 5-15 years, between December 1993 and April 1996, in our institution. The indications were complete AV block with severe bradycardia in 5 patients, second-degree or complete AV block following congenital heart surgery in 3, complete AV block with long QT syndrome in 1, and second-degree AV block and syncope in 1. There were no complications related to the procedure in any case. P and R wave amplitudes were measured and thresholds were determined intraoperatively on all patients. Amplitudes and thresholds were remeasured on seven patients with a mean follow-up of 17 months; Holter monitors were performed on seven patients with mean follow-up of 16 months. P and R wave amplitudes were generally diminished at follow-up compared to initial values but remained within an acceptable range for all patients. Four patients required reprogramming after pacemaker insertion, 1 received an atrial lead for dual chamber pacing, 1 required reposition for lead dislodgment, and 1 patient required a new lead for an inadequate ventricular pacing threshold. No patient had evidence of failure to sense or capture as evaluated by Holter monitoring at last follow-up. Single lead VDD pacing systems can be successfully used in properly selected children with high degree or complete AV block with normal sinus node function.

Atrial flutter in a pediatric patient in the immediate period after the Fontan procedure: control with oral propafenone.

A 2-year-old male underwent Fontan procedure as the third stage palliative repair for hypolastic left heart syndrome. On postoperative day number 15, he developed atrial flutter unresponsive to multiple attempts at cardioversion and therapeutic levels of digoxin and procainamide. Oral propafenone was administered and within 6 hours no further arrhythmias or side effects were noted. This case suggests that oral propafenone can be used with efficacy for the treatment of atrial flutter encountered in the immediate postoperative period following the Fontan procedure.

The representation of brightness in primary visual cortex.

Although neurons in primary visual cortex are sensitive to the spatial distribution and intensity of light, their responses have not been thought to correlate with the perception of brightness. Indeed, primary visual cortex is often described as an initial processing stage that sends information to higher cortical areas where perception of brightness, color, and form occurs. However, a significant percentage of neurons in primary visual cortex were shown to respond in a manner correlated with perceived brightness, rather than responding strictly to the light level in the receptive fields of the cells. This finding suggests that even at the first stage of visual cortical processing, spatial integration of information yields perceptual qualities that are only indirectly related to the pattern of illumination of the retina.

Temporal limits of brightness induction and mechanisms of brightness perception.

The luminance of a squarewave grating was modulated in a manner such that every other stripe temporally varied between bright and dark and the intervening stripes had constant luminance. This produces brightness induction in the constant stripes, roughly in antiphase to the luminance modulation. We used this stimulus as a probe to explore the temporal properties of brightness induction and the mechanisms determining perceived brightness. Over a range of spatial frequencies we measured: (1) the highest temporal frequency at which brightness induction occurs; (2) the magnitude of induced brightness; and (3) the temporal phase of the induced brightness modulation. We find that brightness induction ceases with luminance modulation above a cutoff temporal frequency that depends on spatial frequency. The magnitude of induced brightness modulation is greatest at low spatial frequencies and low temporal frequencies. Induced brightness lags behind the luminance modulation and this phase lag increases as spatial frequency decreases. All of these findings can be understood as consequences of an induction process that takes longer to complete as the induction region increases in size.

Feature-specific effects of selective visual attention.

Four experiments were conducted to quantify the effect of performing a foveal discrimination task on sensitivity for a peripheral grating. The observer's primary task was to discriminate either the spatial frequency or orientation of successive foveal Gabor patches. On a third of the trials they also performed a secondary task to detect the presence of a near-threshold grating in the periphery. We find that sensitivity for detection of the peripheral grating depends on the similarity of the spatial frequencies and orientations between the foveal and peripheral stimuli. Importantly, sensitivity is also affected by which feature is being discriminated in the central task. Because the detectability of the peripheral grating is different when different features of the central stimuli are discriminated, we suggest that the effects on sensitivity are due to feature-specific attention and not simply to passive interactions between filters with similar tuning properties.

Malignant wide complex tachycardia after adenosine administration to a postoperative pediatric patient with congenital heart disease.

Adenosine has become the treatment of choice for paroxysmal supraventricular tachycardia because of its safety and efficacy. There have been no reports of malignant arrhythmias occurring after adenosine administration. This case report presents the occurrence of a malignant wide complex tachycardia after intravenous adenosine administration in a 10-year-old boy 2 days after a Fontan procedure. Thus the administration of adenosine in a critically ill postoperative patient can have morbidity or potential mortality and must be monitored closely.

Usefulness of intermittent monitoring of mixed venous oxygen saturation after stage I palliation for hypoplastic left heart syndrome.

Most deaths after stage I palliation for hypoplastic left heart syndrome have occurred within the first 24 hours after surgery. Efforts to improve 1-day survival should therefore have significant impact on improving overall survival. Early death has most often been attributed to low cardiac output and abnormalities of pulmonary to systemic flow ratio (Qp/Qs). Thirteen infants underwent stage I palliation and had a catheter inserted in the high superior vena cava (SVC) for intermittent measurement of SVC oxygen saturation. Calculation of Qp/Qs was achieved using SVC saturation as a mixed venous oxygen saturation, and estimating pulmonary venous oxygen saturation. Eleven patients survived, and 2 patients died within the first 24 hours. Abnormalities in Qp/Qs were noted in 12 of 13 patients after operation. In 10 of these 12 patients, there was a high Qp/Qs, which has been associated with poor outcome. High Qp/Qs was noted even in patients with acceptable arterial oxygen saturations (< 85%). SVC saturation increased in all survivors during the first 24 hours, and was associated with a decrease in Qp/Qs. Measurement of SVC oxygen saturation appears to be a valuable adjuvant in the postoperative management of infants after stage I palliation of hypoplastic left heart syndrome. Major abnormalities in Qp/Qs can be detected even with acceptable arterial saturations. With this information, early ventilator/pharmaceutical adjustments can be made which may improve stage I survival.